FLUVIAL LANDSCAPES OF THE CRETACEOUS: INSIGHTS FROM STABLE ISOTOPE GEOCHEMISTRY, SEDIMENTOLOGY AND TAPHONOMY

The Kaiparowits Formation of southern Utah preserves one of the best records of late Cretaceous plant and animal life in western North America. Sedimentological studies indicate these organisms occupied fluvial landscapes of the Sevier foreland that were low in relief and frequently inundated, while climate model and simulations and oxygen isotope data indicate that monsoonal precipitation was associated with flooding of the landscape. Carbon isotope data from hadrosaur tooth enamel indicates that forest structure varied over these landscapes, with hadrosaurs partitioning resources from a mix of open and closed forest canopies. What remains unknown is detailed surface hydrology of these landscapes, and how surface flooding and subsequent draining varies over small spatial scales and in turn how such variations correspond to differences in forest structure.

In this study, oxygen isotope ratios of hadrosaur tooth enamel, fish scales, bivalve shells, pond carbonates and pedogenic carbonates are used to infer the ratio of surface waters over different parts of the paleo-floodplain. Bivalve data indicate there are large rivers recharged by upland precipitation plus varying amounts of lower elevation precipitation, while hadrosaur, fish and pedogenic carbonate data indicate that smaller streams were present that were recharged only by low elevation precipitation. Bivalve, fish and pond micrite data also point to existence of lakes that were recharged by a mix of stream and river water, presumably during seasonal flooding events analogous to those taking place in southeast Asia today.

Organic content of sediment and carbon isotope ratios of paleosol carbonate and of hadrosaur dentine from different sites indicate that soils along lake margins were variably saturated, with those closer to the margin being saturated for a longer period of time compared to more distal localities. The more saturated localities are also the ones associated with closed canopy forest structures, which demonstrates the importance of fluvial systems in influencing the distribution of plants over Kaiparowits landscapes.